PROJECT SUMMARY: Dengue and yellow fever virus are significant human pathogens that cause hemorrhagic fever and fatalities. Currently, there are no approved antivirals for these viruses. The development of antivirals for RNA viruses has proven to be particularly difficult due to the rapid emergence of drug resistant viruses. This is primarily due to the error prone nature of the polymerase, which results in rapid evolution of the virus under selective pressure. Targeting a host factor for which the virus does not have any genetic control over should decrease or prevent development of resistant mutants either because the virus cannot function without the host factor or because more mutations are required to confer resistance. Towards this goal we have developed a robust method to identify host factors that are bound to the viral RNA during viral amplification in cell culture. We will use this method to identify host factors that are bound to the dengue virus and yellow fever virus RNA. We will identify which host factors will be good targets to develop antiviral targets. In addition, we will also determine the step in the viral life cycle the host factors participate in and the protein-protein and protein-viral RNA interactions for that host factor during viral infection. These studies will address the mechanism of the host factor in viral amplification and enable development of a high-throughput assay fro inhibitors of the host factor.